The use of photopolymerizable recording materials for the production of flexographic printing plates on which the printing surface is produced by imagewise exposure of a light-sensitive layer and subsequent removal of the unexposed portions of the layer is well known in the art. In this art area, solid photopolymerizable materials are differentiated from those on which the imagewise exposure is made in liquid systems. In addition, there are recording materials that are aqueous-developable and those that can be developed with organic solvents.
U.S. Pat. No. 4,238,560 discloses, for example, aqueous-developable printing plates comprising a support, a photosensitive layer, and a matting layer. In development with aqueous solutions, the entire matting layer is removed as well as the non-image portions of the photosensitive layer. In addition to the actual matting material, the matting layer contains rosin or rosin esters, which, as described in Ullmann's "Encyclopedia of Chemical Technology", 4th edition, volume 12, pages 530-536, Verlag Chemie, Weinheim, 1976, are used for their high pigment compatibility in paints and printing inks.
Compared to solvent-developable materials, a particular shortcoming of aqueous-developable printing plates is their low resistance to alcoholic and aqueous printing inks, which swell the printing surface and thereby cause defective printing.
Examples of solid, solvent-developable recording materials are disclosed in DE-C3 22 15 090 (U.S. Pat. Nos. 4,323,636; 4,323,637; 4,369,246; 4,423,135) U.S. Pat. Nos. 4,266,005, 4,320,188, and 4,430,417. Such photopolymerizable printing plates usually comprise a support, optionally an adhesive or other underlayer, a photopolymerizable layer containing at least one polymeric binder, at least one ethylenically unsaturated monomer that can be addition-polymerized by actinic radiation, and a photoinitiator or photoinitiator system, and a cover element comprising a flexible, polymeric film soluble in the developer solvent, and a separable cover sheet.
A preferred method for preparing such multilayer, photopolymerizable printing plates is a process in which a previously extruded photopolymerizable composition is fed into the nip of a calender and calendered between a support and a cover element, thus forming a photopolymerizable layer between them.
EP-B1 00 84 851 (U.S. Pat. Nos. 4,427,759 and 4,460,675), discloses preparing a multilayer, photopolymerizable printing element having an elastomeric layer between the flexible polymeric film of the cover element and the photopolymerizable layer. The elastomeric layer contains an elastomeric binder, and as optional constituents, a second binder, a dye, one or more monomers, and a photoinitiator.
However, flexographic printing plates prepared from the described solvent-developable materials often have various deficiencies, among which uneven printing quality is especially noteworthy. Thus, with alcoholic and aqueous flexographic printing inks, unsatisfactory printing results are obtained, because inadequate ink transfer in printing leads to inking defects. The transfer of 5% dots is particularly prone to defects in the use of such printing inks.
Accordingly, the object of the present invention is to eliminate the above-cited printing problems and make available a solid, photopolymerizable printing element for preparing flexographic printing plates having improved ink transfer. In addition, the usual washoff development with organic solvents should be feasible. At the same time, other properties of the photopolymerizable printing plates should not be affected adversely. For example, reduction in photosensitivity should be avoided. Moreover, the flexographic printing plates should not swell in printing inks, because such swelling causes undesirable dot growth and consequently, defective printing.
Surprisingly, the aforementioned objectives are achieved by using a photopolymerizable printing element comprising a support (a), a photopolymerizable layer (b) containing at least one elastomeric binder, at least one ethylenically unsaturated monomer addition-polymerizable by actinic radiation, and a photoinitiator or photoinitiator system, an elastomeric layer (c) containing at least one thermoplastic, elastomeric block copolymer, and a cover sheet (d).